As is known to those skilled in the art, iron ore is one of the most widely produced and consumed mineral substances in the world. In economic terms, the iron mining industry is vitally important for the countries that produce it, such as Brazil, for example, where iron ore accounts for as much 15% of the country's total mineral production and a significant portion of the iron ore produced is destined for the foreign market.
Commercially, iron ores produced for export are products found in the forms of natural ore, granulated ore, sinter feed, pellet feed and agglomerated ore, i.e. pellets.
Generally speaking, in order to obtain iron ores in the form of pellets it is necessary to subject dressed ore to the pelletization process, whose aim is to agglomerate ore fines, coal and other minerals into a spherical shape and subsequently fire them, i.e. sinter these pellets in special furnaces.
More specifically, the pelletization process comprises five stages: 1) thickening, in which concentrated ore pulp is collected in specific tanks (thickeners) that increase the percentage of pulp solids by 70% to 75%; 2) filtration, in which vacuum filters and pumps work together to remove water from the iron ore pulp, reducing the moisture content to an appropriate level; 3) Mixing or crushing, in which the product of filtration (pellet feed) is stored in appropriate silos and mixed with other ores in mechanical stirrers in order to create physical and chemical conditions that are favorable to pellet formation; 4) balling, in which unfired (“green”) pellets are formed on balling disks; and 5) hardening or firing, in which the unfired or green pellets formed on the balling disks are subjected to careful thermal treatment in furnaces, giving them the physical and mechanical strength appropriate for handling and transportation to the consuming market.
Thus, as taught above, the formation of unfired pellets, also known as “green” pellets, occurs on the balling disks.
In said devices, the material (iron ore) is sprayed with a certain amount of water (8-9% moisture). As the ore comes into contact with the circular surface of the disk, which is maintained in a slightly inclined position and rotates at a given speed, and because said ore is permanently driven to the ascending portion of the disk, friction between the ore granules starts to form pellets, which, through repeated, constant rotation, results in the addition of material until a specific desired size is achieved.
In this stage, rigorous control of the agglomeration process is fundamental, as a basic condition for obtaining a final product that meets market requirements regarding product quality, since granulometric range control is a key requirement for the reduction process that is subsequently performed by steel mills.
However, as is known to those skilled in the art, it is not always possible to maintain a consistently high quality level of the pellets being formed. That is because, during production of unfired pellets on said balling disks, a number of inconveniences occur that prevent obtainment of ore pellets whose composition and dimensions are uniform and suited to the requirements of the consuming market.
Among these inconveniences, there stands out the formation of pellets that are larger than the standard sizes. These large pellets are known in the steel industry as “oversize pellets.”
It is known that the formation of oversize pellets is common, since they result from the very movement of the balling disk. In other words: the moist pellets accumulate large amounts of agglutinant material, where the ore nuclei that form oversize pellets collide with the ideally sized pellets, causing the latter ones to break. Consequently, by incorporating these fractions, the volume of the material to be fed back to the balling circuit increases, which tends to interfere with process productivity, thus reducing the output and increasing production costs.
Thus, it becomes necessary for operators to frequently intervene during the pelletization process (balling stage) to remove these undesired oversize pellets, often using spears and shovels.
Even though it is possible to remove oversize pellets, operators regard this task as laborious and physically demanding, since they are constantly subjected to ergonomically incorrect positions which, over time, tend to do harm to their health.
Another reason for this oversize pellet removal procedure is considered inappropriate relates to the fact that pellet quality control by operators is subject to errors, due to the large number of balling disks to be monitored and also because the balling process is virtually uninterrupted.